(1) Field of the Invention
The present invention relates to a method for the treatment of sludges such as sewage sludges, night soils, water supply sludges, sludges deposited in bottoms of rivers, lakes, harbors and seas and industrial waste water sludges. More particularly, the present invention relates to a sludge treatment method suitable for coagulation solids and solid-liquid separation in sludges containing organic materials and fine particles of soils.
(2) Description of the Prior Art
As means for disposal of sludges, there is ordinarily adopted a method in which a coagulant is added to a sludge to coagulate solids and solid-liquid separation is carried out or the coagulated solids are mechanically dewatered to form a dewatered sludge having a reduced volume, and the dewatered sludge is discarded as it is or the dewatered sludge is discarded after it has been burnt to ashes or solidified by incorporation of a solidifier.
Sludges rich in organic materials, such as sewage sludges, contain large quantities of hydrophilic colloidal substances and have a high water content, and therefore, it is difficult to dewater these sludges directly as they are. Accordingly, a coagulant is added to such sludge to coagulate solids and the sludge is then mechanically dewatered to form a dewatered sludge (hereinafter referred to as "filter cake"). This filter cake is discarded as it is or after it has been burnt. An inorganic coagulant such as ferric chloride or slaked lime is most popularly used as the coagulant and vacuum filtration is adopted for this mechanical dewatering. However, since the inorganic coagulant is added in a relatively large amount to solids in the sludge, the amount of the filter cake to be disposed of is increased. In the case where the filter cake is burnt, large quantities of ashes are formed and the efficiency of combustion of the filter cake is lowered by incorporation of the coagulant. Moreover, at the dewatering step, ferric chloride or calcium carbonate is deposited on a dewatering filter cloth, resulting in reduction of the filtration efficiency.
Furthermore, there has been adopted a method in which an organic polymeric coagulant (hereinafter referred to as "polymeric coagulant") is used as the coagulant to coagulate the sludge and form a floc and the floc is mechanically dewatered. This method is advantageous over the method using an inorganic filler, because the amount of the filter cake and the amount of ashes left after combustion of the filter cake are reduced. However, the water retention ratio of the floc formed by coagulation is high, and since water is not easily separated from the floc, the floc is readily disintegrated. Therefore, vacuum filtration or pressure filtration cannot be adopted for dewatering of this floc, and hence, dewatering of the floc is performed by centrifugal dewatering. However, the water content of the cake is not significantly reduced by centrifugal dewatering. Furthermore, this method using an organic polymeric coagulant is defective in that the treatment expenses are increased.